KIEV/SLOVYANSK/, DONETSK UKRAINE – The Group of Seven rich countries agreed Saturday to impose new sanctions on Russia over the crisis in Ukraine, after Kiev accused Moscow of seeking to trigger a “third world war.”

The G-7 nations, in a joint statement, said they would “move swiftly to impose additional sanctions on Russia,” with an informed source saying the new U.S. sanctions could come as early as Monday.

The G-7 statement praised the “restraint” with which the new government in Kiev had acted in dealing with the pro-Russian gunmen who have seized official buildings in the east of Ukraine.

Earlier, German Chancellor Angela Merkel said EU foreign ministers will meet soon to discuss the issue after speaking by conference call with U.S. President Barack Obama, British Prime Minister David Cameron, French President Francois Hollande and Italian Prime Minister Matteo Renzi.

The G-7 also includes Canada and Japan.

Meanwhile, Ukrainian authorities ratcheted up military operations against pro-Russian rebels in the east and their Cold War-style rhetoric.

“The world hasn’t forgotten the Second World War and Russia wants to start a third world war,” Ukrainian Prime Minister Arseniy Yatsenyuk said.

“Russia’s support for the terrorists in Ukraine constitutes an international crime and we call on the international community to unite against the Russian aggression.”

A Western diplomat voiced concern of a possible Russian move into eastern Ukraine in the coming days, maybe even over the weekend.

“We no longer exclude a Russian military intervention in Ukraine in the coming days,” the diplomatic source said, noting that Russia’s U.N. envoy, Vitaly Churkin, “has been recalled urgently to Moscow” for consultations.

Tensions were further heightened on the ground as the Ukrainian military launched a new offensive to besiege the rebel-held city of Slovyansk and insurgents blew up an army helicopter with a rocket-propelled grenade.

At the entrance to Slovyansk, several members of an Organization for Security and Cooperation in Europe (OSCE) observer mission were detained and taken to the rebel-held security services building, sparking immediate international condemnation.

German Defense Minister Ursula von der Leyen said pro-Russian separatists arrested 13 mission members, including three members of the German Army and an interpreter.

Pro-Russian rebels holding the observers on Saturday accused them of being “NATO spies” and vowed to continue detaining them.

Washington called for the immediate release of the OSCE team and State Department spokeswoman Jen Psaki insisted “there is a strong connection between Russia and these separatists,” carrying out such hostage-takings.

It was not clear what the intent was, but the aircraft could have been testing Ukrainian radar or making a show of force, said the officials, who spoke on condition of anonymity because they were not authorized to talk publicly about the issue.

The flights came as Russia increased military exercises along the Ukraine border, including moving a broad array of fixed wing and rotary aircraft, infantry and armored troops — further inflaming fears of a potential Russian military incursion into Ukraine.

The United States and the European Union have already targeted Russian President Vladimir Putin’s inner circle with visa and asset freezes and imposed sanctions on a key Russian bank.

Western leaders have repeatedly threatened to hit Russia with measures aimed at the wider economy.

However, Obama has signaled the new sanctions will not involve an attempt to target key areas of the Russian economy such as mining, energy and the financial sectors.

U.S. officials have said those measures will only be considered if Russia sends its regular forces across the border into eastern Ukraine.

The White House said that Moscow could “still choose a peaceful resolution to the crisis” by implementing a deal struck in Geneva two weeks ago to defuse the tension.

Ukraine Deputy Foreign Minister Danylo Lubkivsky told reporters at the United Nations that his country will exercise restraint in its operations against pro-Russian separatists.

“The anti-terrorist operation is ongoing, but we are guided by one major idea: we would like to avoid any victims or casualties,” he said.

Kiev announced that its forces are now seeking to “blockade” rebels inside Slovyansk, in a bid to prevent militant reinforcements from arriving and to spare civilian casualties.

Heavily armed troops were seen setting up a checkpoint some 15 km (10 miles) from the town of 110,000 people.

On Thursday, Ukrainian armored vehicles and commandos had made a brief but dramatic incursion into Slovyansk, killing a 22-year-old insurgent.

But the rebels in Slovyansk were defiant Friday, vowing, “We will not surrender the town.”

Just 16 km to the south, at an air base close to the city of Kramatorsk, a rocket-propelled grenade blew up a Ukrainian military helicopter sitting on the tarmac, officials in Kiev said.

The pilot escaped but was wounded.

Russia responded to Ukraine’s military offensive by ordering its troops massed on the border to launch new drills.